U.S. patent number 5,200,146 [Application Number 07/661,553] was granted by the patent office on 1993-04-06 for apparatus for effecting plasma sterilization.
This patent grant is currently assigned to Air Techniques, Inc.. Invention is credited to Claude A. Goodman.
United States Patent |
5,200,146 |
Goodman |
April 6, 1993 |
Apparatus for effecting plasma sterilization
Abstract
There is disclosed an invention for effecting sterilization in a
plasma of low molecular weight aliphatic alcohol, preferably
methanol at pressure levels of from 10 mTorr to 10 Torr at RF
energy levels to establish and maintain a plasma for a time
sufficient to achieve substantial total microbicidal
effectiveness.
Inventors: |
Goodman; Claude A.
(Gaithersburg, MD) |
Assignee: |
Air Techniques, Inc.
(Hicksville, NY)
|
Family
ID: |
24654096 |
Appl.
No.: |
07/661,553 |
Filed: |
February 26, 1991 |
Current U.S.
Class: |
422/23; 422/21;
422/22; 422/28; 422/292; 422/295; 422/297 |
Current CPC
Class: |
A61L
2/14 (20130101) |
Current International
Class: |
A61L
2/02 (20060101); A61L 2/14 (20060101); A61L
002/14 () |
Field of
Search: |
;422/21,22,23,28,292,295,297 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Warden; Robert J.
Assistant Examiner: Santiago; Amalia
Attorney, Agent or Firm: Marn; Louis E.
Claims
What is claimed is:
1. A plasma sterilization assembly, which comprises:
a microwave means defining a microwave chamber;
means for generating RF energy within said microwave chamber;
a sterilization vessel defining a sterilization chamber disposed
inside said microwave chamber;
perforated tray means disposed in said sterilization chamber for
receiving an article to be sterilized;
a storage vessel containing a low molecular weight aliphatic
alcohol;
vacuum pump for evacuating said sterilization chamber;
conduit means providing fluid flow communication between said
sterilization chamber and said vacuum pump;
conduit means for introducing vaporous low molecular weight
aliphatic alcohol into said sterilization chamber from said storage
vessel; and
means for energizing said means for generating said RF energy to a
level to form a plasma of said low molecular weight aliphatic
alcohol and thereby generate micro-molecular concentrations of
microbicidal agents.
2. The plasma sterilization assembly as defined in claim 1 wherein
said means for generating RF frequency is in the range of from
10.sup.5 to 10.sup.15 Hertz. wherein a source of RF energy is
between 0.08 to 1.6 KW.
3. The plasma sterilization assembly as defined in claim 1 or 2
wherein said means for generating is capable of generating RF
energy between 0.8 to 1.6 KW.
4. The plasma sterilization assembly as defined in claim 1 wherein
said vacuum pump maintains a vacuum of from 10 mTorr to 10
Torr.
5. The plasma sterilization assembly as defined in claim 1 and
further including means to modulate said means for generating RF
frequency to maintain a temperature in said sterilization chamber
below about 160.degree. C. to 170.degree. C.
6. The plasma sterilization assembly as defined in claim 1 wherein
said perforated tray is sized to be not evenly divisible by
one-fourth of the wave length of an RF source.
7. The plasma sterilization assembly as defined in claim 1 and
further including cooling means in fluid communication with said
microwave chamber.
8. The plasma sterilization assembly as defined in claim 1 and
further including an electrically conductive shield disposed on
said perforated tray means.
Description
BACKGROUND OF THE INVENTION
1) Field of the Invention
This invention relates to sterilization, and more particularly to a
process and apparatus for effecting plasma sterilization using low
molecular weight aliphatic alcohols, preferably methanol.
2) Description of the Prior Art
There is a growing concern about the spread of infections during
routine healthcare treatments due to public awareness of
transmissible diseases including hepatitis and AIDS, also echoed by
the CDC (Center for Disease Control) and the FDA (Food and Drug
Administration) currently focused on nosocomial infection. A
plethora of sterilization techniques are and have been practiced
for years using heat, sterilization agents and the like, as well as
including the use of plasma sterilization techniques. For instance
in U.S. Pat. No. 3,383,163, there is disclosed the use of an argon
RF plasma to sterilize the interior walls of glass bottles. A
chemically reactive halogen gas RF plasma is disclosed in U.S. Pat.
No. 3,701,628 to achieve sterilization at lower temperatures and
while suitable for glass and plastic bottles, such method is
corrosive to metals.
In U.S. Pat. No. 4,207,286, there is disclosed a method for passing
an aldehyde feed gas RF plasma over objects to achieve surface
sterilization. In U.S. Pat. No. 4,348,357, there is disclosed the
use of pressure/power cycles to convectively force the (oxygen) RF
plasma species into holes, apertures and cavitites as well as to
sterilize objects of irregular shape. In U.S. Pat. No. 4,599,216,
there is disclosed an apparatus for increasing the uniformity of
microwave exposure by rotating dental items through the microwave
field. A microwave adsorber is necessary to reduce the likelihood
of metal objects arcing back to the magnetron, however, long
exposures obtained erratic results--insufficient to justify claims
of sterility.
In U.S. Pat. No. 4,643,876, there is disclosed the use of hydrogen
peroxide (liquid) in the pre-treatment cycle and (vapor) as a
precursor for active species made in an RF plasma. In U.S. Pat. No.
4,756,882 there is described the conversion of residual hydrogen
peroxide into non-toxic decomposition products In U.S. Pat. No.
4,801,427 there is disclosed the use of hydrogen, oxygen, nitrogen,
halogens, organohalogens, inorganic halogens, inorganic
oxyhalogenated compounds, inert gases and mixtures to produce RF
plasmas to sterilize medical instruments. In U.S. Pat. No.
4,804,431, there is described the modification of a conventional
microwave oven to produce oxygen or argon plasmas for etching and
cleaning.
Traditional sterilization methods include: ethylene oxide gas which
leaves toxic residues; dry heat which may corrode metal; steam
autoclaving dulling blades and points; chemical vapor (e.g.
formaldehyde and gluteraldehyde); gamma, x-ray and electron
beam/curtain radiation which is only suitable for batch or
production line processing, etc. While all such techniques have
achieved satisfactory levels of sterilization for particular uses
of the thus sterilized item, such techniques suffer from
universality to the materials of construction of items to be
sterilized as well as requirements to store dangerous chemical
sterilants with concomitant handling difficulties. Additionally,
such techniques are ineffective in situations requiring rapid
sterilization, e.g. in a surgical procedure when a critical
instrument is unsterile, dropped or otherwise contaminated.
OBJECTS OF THE INVENTION
An object of the present invention is to provide a plasma
sterilization process and apparatus of improved effectiveness.
Another object of the present invention is to provide a plasma
sterilization process and apparatus eliminating the storage and
handling of dangerous chemical sterilant agents.
A further object of the present invention is to provide a plasma
sterilization process and apparatus of universality of application
to articles being sterilized.
Still another object of the present invention is to provide a
plasma sterilization process and apparatus of improved
effectiveness for the rapid sterilization of critical items during
surgical protocol.
Yet another object of the present invention is to provide a plasma
sterilization process and apparatus not adversely affecting the
physical or chemical properties of articles being
decontaminated.
SUMMARY OF THE INVENTION
These and other objects of the present invention are achieved by a
process and apparatus for effecting plasma sterilization using a
low molecular weight aliphatic alcohol, preferably methanol, at
pressure levels of from 10 mTorr to 10 Torr at RF energy levels to
establish and maintain a plasma for a time sufficient to achieve
substantially total microbicidal effectiveness.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantages of the present invention will become
more apparent from the following detailed description thereof when
taken with the accompanying drawings wherein:
FIG. 1 is a schematic drawing of the apparatus of the present
invention;
FIG. 2 is a graph illustrating kill rate as a function of power in
the Kill versus Time relationship;
FIG. 3 is a graph of the same date as FIG. 2, normalized to total
applied energy, illustrating constant energy effects in Kill vs.
Energy relationship; and
FIG. 4 is a graph comparing the Kill kinetics of air and methanol
plasmas illustrating superiority of methanol plasma sterilization
in Kill vs. Energy relationship.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 1, there is illustrated a plasma
sterilization assembly, generally indicated as 10, comprised of a
multimode reflective cavity or microwave oven 12 defining a
microwave chamber 14 and including a door member 16. A
horizontally-disposed cylindrically-shaped vessel 18 defining a
sterilization chamber 20 is positioned within the microwave chamber
14 and is provided with a horizontally-disposed inert tray member
22 positioned therein. The microwave oven 12 is provided with a
wave guide member 24 electromagnetically coupled to a source 26 of
RF energy as known to one skilled in the art, connected to an
electrical power supply (not shown) and modulated via a control
panel 28 via conductor 65. The microwave oven 12 of the sterilizing
assembly 10 is provided with a cooling fan 30 in fluid
communication by a conduit 32 with the chamber 14 of the microwave
oven 12 including a vent conduit 34 as more fully hereinafter
discussed.
The vessel 18 is formed of a highly radiation-transparent material,
such as quartz, Pyrex.RTM. glass or the like. The vessel 18 is
provided with a cooperating cover member 38 including appropriate
gasket member 40 to permit evacuation of the sterilization chamber
20 to below atmospheric pressure levels or a vacuum of from 10
mTorr to about 10 Torr. The cover member 38 is formed of an inert
material, such as aluminum and is of a thickness sufficient to
support the hereinabove disclosed below atmospheric pressure levels
or vacuum. The tray member 22 is likewise formed of an inert
material, such as aluminum, and is formed with a plurality of
orifices 42 to permit effective plasma formation about the article
(not shown) being sterilized. The dimensions of the tray member 22
are sized to be compatible with the wave form of the generated RF
energy as more fully hereinafter discussed. An electrically
conductive shield or Faraday cage 44, as known to one skilled in
the art, may optionally be employed in the process and apparatus of
the present invention.
A closed end 46 of the vessel 18 within the microwave oven 12 is
provided with a conduit 48 under the control of valve 50 in fluid
flow relationship to a storage vessel 52 for the aliphatic
hydrocarbon alcohols, preferably methanol, and a conduit 54
including a vacuum pressure gauge 56 in fluid flow relationship
with the suction side of a vacuum pump 58. The conduit 54 is in
fluid flow relationship with the atmosphere via a conduit 60 under
the control of a valve 62. Electronic sensor elements 64 are
positioned within the vessel 18 and connected via conductor 66 to
the control panel 28 including monitoring, energizing and process
control members (not shown).
In operation, an article (not shown) to be sterilized is preferably
first treated to remove gross detritus and possibly even subjected
to ultrasonic cleaning techniques. The thus cleaned article is
dipped into a low molecular weight aliphatic alcohol having 6 or
less carbon atoms, and preferably methanol, as more fully
hereinafter discussed. The article is then placed on the perforated
tray 22 within the chamber 20 of the sterilization vessel 18. The
door or cover 38 is positioned over the opening of the chamber 20
to thereby enclose the microwave oven 12. The door member 16 of the
microwave oven 12 is thereafter closed thereby deactivating any
upset switch permitting further operation of the apparatus of the
present invention.
The cooling fan 30 is thus activated followed by activation of the
vacuum pump 58 is activated to reduce pressure within the chamber
20 to less than about 100 mTorr. whereupon the microwave source 26
is energized to ignite a plasma, generally at a RF energy level of
about 0.08 to 1.6 KW, as understood by one skilled in the art.
During pressure reduction, methanol vapor is introduced by line 48
from the storage vessel 52 to maintain and achieve a minimum
concentration of at least about 95 volume percent. Since oxygen is
a known corrosive agent for some metallic instruments, it is
desirable to achieve as high as possible concentration level of
methanol vapor on the sterilization chamber 20 prior to plasma
ignition.
Plasma generation is maintained for a time sufficient to reach an
acceptable end point of sterilization, such as from 1 to about 60
minutes as a function of total energy. Generally, the microwave
source 26 is operated to generate an electromagnetic field power
density of from 0.01 Watts/cm.sup.3 to about 0.2 Watts/cm.sup.3 for
a time sufficient to provide a total energy input of at least about
150 KJoules. Preferably, the radiated energy of the electromagnetic
field is at a frequency of from about 10.sup.5 to 10.sup.15 Hertz.
The electromagnetic field may be pulsed and/or half or full wave
rectified or of a low ripple three phase source. In this context
the tray 22 width and length are sized so as not to be evenly
divisible by one-fourth of the wave length of the RF source.
After sufficient energy has been delivered to the sterilization
assembly 10, the RF energy source 26 and the vacuum pump 58 are
discontinued and the vessel 18 is vented to atmosphere via conduits
54 and 60 under the control of valve 62.
It will be understood by one skilled in the art that heat may be
generated during the process of the present invention, and as such
effects the rate of microbicidal results. Should heat be generated,
it is desired not to permit elevated temperature levels above about
160.degree. to 170.degree. C. In the event temperatures should
reach about 160.degree. to 170.degree. C., the control panel 28
modulates the RF energy source 26 to maintain such temperature
level.
Operation of the process of the present invention is illustrated in
the graphs of FIGS. 2, 3 and 4. It is evident that effectiveness or
kill is time dependent as a function of power level. Thus, the
curves of FIG. 2 illustrate the characteristic doubling of slope as
the power is doubled. Variance between the 80 sec. 300 w samples is
believed due to non-homogenity of plasma at low powers in large
sterilization chambers.
In FIG. 3, the curves essentially illustrate a constant kill rate
at constant energy input regardless of the rate of applied energy
over two doubling of rate. Thus, kill or totallity of microbicidal
effect is proportional to the total energy applied to the system
provided adequate time for the diffusion process.
As readily understood by one skilled in the art, effective
sterilization is the totality of destruction or strong microbicidal
effect to zero survival, although as a practical matter, an
impossible state to validate absolutely. The FDA has provided a
guideline for a testing protocol for determining microbicidal
effectiveness referencing a shape of a survivor curve, a minimum
contact time and a D value determined from experimental results of
both quantitative kill kinetics and fraction positive/negative
probability of sterilization analyses. Thus, spericial activity, as
recommended by the FDA, is a sterilizing process reliable to assure
a probability of survival of less than one organism out of
1,000,000 or alternately less than 1.times.10.sup.-6. In FIG. 4,
these analyses are illustrated for air and methanol plasma
processes.
While the microbicidal mechanism is not completely understood, it
is believed that extremely potent microbicidal agents in micromolar
concentrations are synthesized from the aliphatic alcohol,
particularly from methanol. With the alcohol there is obviated the
need to stock and handle bulk quantities of dangerous chemical
sterilants, e.g. ethylene oxide, formaldehyde, etc. Additionally,
the alcohols and particularly methanol may be conveniently used in
the precleaning/drying process as a solvent. The use of
substantially pure methanol compared with the C.sub.2 + aliphatic
alcolhols is preferred since undesired polymerization products may
be formed with the heavier alcohols and be deposited on the
instruments being sterilized. While processing conditions may be
used which minimize such polymerization conditions, process control
may be unduly complicated compared with the simplicity of using
methanol.
It is understood by one skilled in the art that methanol may be
admixed with water, hydrogen peroxide, and/or other of the low
molecular weight organic compounds although the use of
substantially pure methanol is preferred. Since hydrogen is
believed to be generated during plasma generation, the presence of
minor amounts of oxygen scavenges hydrogen thereby increasing the
yields of micromolar concentrations of the potent microbicidal
agents. Thus, the addition of minor quantities of less than about
0.3 m of an oxygen containing material during plasma generation may
enhance the sterilization process of the present invention.
The process of the present invention is clean, rapid and energy
efficient compared with the existing chemical, heat, etc.
technologies. Methanol, the preferred low molecular weight
aliphatic alcohol is not corrosive to articles being sterilized as
well as being safer to the operators and the environment. Still
further, while the process of the present invention is described
with the generation of a plasma within the sterilization chamber
about the article being sterilized, plasma generation may be
effected in another microwave assembly and passed to the
sterilization chamber of the present invention.
While the invention has been described in connection with an
exemplary embodiment thereof, it will be understood that many
modifications will be apparent to those of ordinary skill in the
art; and that this application is intended to cover any adaptations
of variations thereof. Therefore, it is manifestly intended that
this invention be only limited by the claims and the equivalents
thereof.
* * * * *